In upcoming sci-fi flick Source Code, an Iraq War veteran uses “time reassignment” technology to crack open alternate realities in an effort to stop a terrorist from blowing up Chicago. While the premise sounds outlandish, it’s actually based on principles of quantum physics, according to Source Code scribe Ben Ripley.

[eventbug]”More and more, modern physics is becoming comfortable with the possibility of parallel universes,” Ripley said in an e-mail interview with Wired.com. “Source Code imagines a practical application for them.”

The ancient notion of parallel universes, rooted in fables and mythic fantasy tales, continues to play a key role in modern movies and TV shows like Fringe and J.J. Abrams’ Star Trek reboot. But in Source Code, director Duncan Jones (Moon) takes the alt-world conceit in a new direction, with actor Jake Gyllenhaal portraying slipstreaming soldier Colter Stevens.

Screenwriter Ripley said he took pains to avoid sci-fi’s typical time-travel paradoxes in the movie, which screens next month at the South by Southwest film festival in Austin, Texas. In Source Code, he said, “What’s done is done and cannot be undone.”

(Spoiler alert: Minor plot points follow.)

“‘Time reassignment’ respects this boundary by creating a parallel world whose past we may visit and freely disturb, since those actions have no causal impact on our own reality,” Ripley said. “Whatever Colter does in the Source Code changes absolutely nothing that has already happened in the primary reality.”

Colter’s assignment finds him revisiting an eight-minute time loop animated by the brain activity of victims killed during an earlier train attack. While on his Groundhog Day–style mission, Colter searches for clues that might lead to the identity of the terrorist behind the bombings.

While the mechanism sounds utterly fantastic, astrophysicist Michio Kaku said it’s not that far from current thinking.

“What you see in Source Code is highly improbable, but it’s possible.”

“What you see in Source Code is highly improbable, but in principle, it’s possible,” Kaku told Wired.com.

Citing the “many worlds” theory of quantum mechanics proposed by Hugh Everett in 1957, Physics of the Impossible author Kaku said the existence of parallel universes is widely accepted.

The tricky part is crossing over.

“It requires coherence, meaning every single atom in one world must resonate in phase with its counterpart in the other universe,” said Kaku, who teaches string theory at the City College of New York and hosts Science Channel’s March 6 show, The Science Behind Firefly.

Kaku’s students calculate that it would take 65 million years to rematerialize yourself on the other side of a brick wall. That would make the odds against catching a terrorist on a train during eight minutes of alternate-universe sleuthing astronomical.

“For all intents and purposes, it’s not going to happen,” Kaku said. “In our lifetime, or for that matter in our universe’s lifetime.”

Accessing Corpses’ Brains

Aside from Source Code‘s multiple-reality paradigm, the movie hinges on the notion that Gyllenhaal’s Colter can retrace events by accessing mental impressions formed in the brain during the victims’ final eight minutes of life.

Ripley frames the scenario in metaphorical terms: “[Consider] the idea of the afterimage. It exists visually: that brief silhouette of a lit window that lingers on the back of your eyes after you close them. It exists aurally: your ears ringing after a rock concert. It exists emotionally: the reverberation of a terrible argument you have with someone for hours afterward. Taken together — the permanence of our stored memories and the echoing afterimage — we arrive at the eight-minute loop.”

“It’s inconceivable that you could trace brain activity back eight minutes after death.”

“The premise is highly implausible, and that’s an understatement,” he told Wired.com by phone. “The notion that we could recover [activity generated by] those billions of neurons after a person has died is inconceivable. We can’t even do that in a monkey or rat that’s alive. It’s inconceivable that you could trace brain activity back eight minutes after death.”

Capturing extensive memories from a dead brain may be a scientific non-starter, but there is, strangely enough, precedent for recovering posthumous visual impressions. As researchers Russell and Karen De Valois reported in their Spatial Vision study, a monkey injected with radioactive tracers was shown a grid pattern just before its death. They could then detect a crude impression of the grid embedded in the dead animal’s brain tissue.

“The grid pattern that the monkey saw in the last seconds of its life was located in the first cortical visual area of the brain,” Biederman said, “so you’re essentially getting a map of the activity that the monkey experienced when it was looking out at the world.”

Stranger still: Scientists could conceivably identify a terrorist by studying the unique synaptic patterns firing in the hippocampus region of a witness’ brain. In research conducted by University of California at San Diego scientist Susumu Tonegawa, test subjects presented with an image of Jennifer Aniston generated a distinctive array of synaptic spikes pegged specifically to the actress’ face.

Source Code‘s dramatic speculations may rip a sizable hole in the fabric of the space-time-believability continuum, but Ripley says his tale is ultimately driven by character, not science.

“In 2005, I was looking for a science fiction device to tell a nonlinear story like Rashomon,” he said. “The narrative took twists and turns from there and a lot of the development was about stripping away complexities and frankly removing much of the scientific explanation. Source Code is a character mystery and the thriller plot is simply the engine.”

Still, Ripley’s not giving up on the “I see dead people’s brains” story hook.

“We don’t yet know enough about the brain to verify the existence of such a loop — nor determine its duration — but it may well exist,” he said. “In any event, it makes a hell of a narrative device.”

Source Code debuts March 11 at the South by Southwest film festival and opens wide April 1.